1. Field of the Invention
The present invention relates to improving the uniformity of the response of a sensor to a mechanical interaction, in particular to improving the uniformity of the sensitivity of a sensor having at least one conductive textile knitted layer.
2. Description of the Related Art
In many applications of manually operable touch sensors, the sensor is required to be flexible and sensitive to applied pressure within predetermined tolerances. A type of fabric touch sensor having a three layer construction comprises two outer conductive textile layers and a central separator layer defining a plurality of apertures. The separator layer is configured to space the conductive textile layers apart when no pressure is applied to the sensor, and to allow electrical contact between the layers under a mechanical interaction.
A problem with this type of textile sensor is that the frequency of undesirable triggering of the sensor may be unacceptable for some applications. Undesirable triggering may be caused by bending or flexing of the sensor, or by internal forces within the sensor arising from deviations from the sensor pattern during manufacture, creases or other set within one or more layers accrued during manufacture or use of the sensor.
US patent publication U.S. Pat. No. 4,659,873 discloses a textile sensor comprising two outer conductive textile layers and a central insulating separator layer, in which the layers are stretched across a frame such that the layers are held flat across the apertures of the separator layer. This arrangement is not suitable for applications of touch sensors in which flexibility is required, and the frame may impart unacceptable variations in sensitivity to mechanical interactions at different locations across the sensing area.
International patent publication WO 00/072239 describes a type of textile sensor constructed from five layers, which provides improved sensitivity and resistance to undesirable triggering. The cost of production of this more complex sensor, however, is considered to diminish the viable range of applications of the sensor.
It is thus desirable to provide a sensor that is flexible, displays uniform sensitivity and is economical to manufacture.